Suspect the solution here is to use a different field. Right now, we require absolute URLs and you want a RelativeURLField, which carries a whole bunch of bonus assumptions or requirements (e.g. relative to what?) that make it not usable in all the same places that URLField is.

Since subclassing fields is fairly easily, encouraged and supported, let's not modify the existing field. Whether it's worth including the subclass in Django itself is a conversation to be had in the future after the subfield has existed for a while and adopted a massive userbase.

For now, though, closing as wontfix, since it appears to work as intended and as a feature change to an existing field, things get complicated fast. If all the problems are overcome when development is done, it's worth a discussion on django-dev, but that is also for the future.